I put this in the other thread, but ah, what the hell.
From A Short History of Nearly Everything by Bill Bryson:
“Statistically speaking the probability that there are other thinking beings out there is good. Nobody knows how many stars there are in the Milky Way—estimates range from 100 billion or so to perhaps 400 billion—and the milky Way is just one of 140 billion or so other galaxies, many of them even larger than outs. In the 1960’s a professor at Cornell, named Frank Drake, excited by such a whopping numbers, worked out a famous equation designed to calculate the chances of advanced life in the cosmos based on a series on diminishing probabilities.
Under Drake’s equation you divide the number of stars in a selected protion of the universe by the number of stars that are likely to have panetary systems; divide that by the number of planetary systems that could theoretically support life; divide that by the number of which life, having arisen, advanced to a state of intelligence; and so on. At each such division, the number shrinks colossally—yet even with the most conservative inputs the number of advanced ciliizations just in the Milky Way always works out to be somewhere in the millions.
What an interesting and exciting thought. We may be only one of millions of advanced civilizations. Unfortunately, space being spacious, the average distance between any two of these bivilizations is reckoned to be at least two hundred light-years, which is a great deal more than merely saying it makes it sound. It means for a start that even if these beings know we are here and are somehow able to see us in their telescopes, they’re watching light that left the Earth two hundred years ago. So they’re not seeing you and me. They’re watching the French Revolutions and Thomas Jefferson and people in silk stockings and powdered wigs. (....) So even if we are not really alone, in all practical terms we are. Carl Sagan calculated the number of probably planets in the universe at large at 10 billion trillion—a number vastly beyond imagining. But what is equally beyond imagining is the amount of space through which they are lightly scattered.”